1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus usually includes a fixing device in which a toner, made of resin, a magnetic material, a coloring agent and the like, that is electrostatically held on a transfer material is fused and fixed by applying heat and pressure while grasping and conveying the transfer material and the toner at a pressing portion (nip portion) between heating means (a roller, an endless belt, or the like) and pressing means (a roller, an endless belt, or the like).
The configuration and the operation of a heating fixing unit, serving as a conventional fixing device, and the configuration and the operation of a color copier, serving as an image forming apparatus having the heating fixing unit will now be described with reference to FIGS. 4 and 5.
FIG. 4 is a cross-sectional view illustrating an original-reader unit 50, an original-reading device 52, and a principal portion of a printer unit 20 of a color copier.
The original-reader unit 50 provides electronic data by reading an original placed on an upper surface thereof, and includes a first mirror unit 50a, a second mirror unit 50b, a lens 50c, a CCD (charge coupled device) 51c, and a platen 50e. 
The printer unit 20 for printing electronic data obtained by the original-reader unit 50 includes a sheet feeding unit 40, an image forming unit disposed above the sheet feeding unit 40, a heating fixing unit 36, an exhaust fan 55 for preventing heat transfer from the heating fixing unit 36 to the image forming unit and exhausting air within the apparatus, and a sheet discharge unit having a discharged-sheet tray 45.
The image forming unit includes a photosensitive drum 30 capable of rotating by a driving mechanism, around which a rotating developing member 34 incorporating a magenta developing unit 34a, a cyan developing unit 34b, a yellow developing unit 34c, and a black developing unit 34d, an intermediate transfer belt 35, a belt cleaner 35a that is usually separated from the intermediate transfer belt 35 except immediately after image formation, a cleaner 31, charging means 32, and an optical scanning device 33 for projecting a laser beam onto the photosensitive drum 30 are disposed.
Next, the operation of the color copier having the above-described configuration will be described.
The operator intending to copy an original using the color copier first mounts the original on an original tray 52a, and then causes the color copier to operate by depressing a start key (not shown) provided on the original-reader unit 50.
The color copier that has started the operation feeds the original onto the upper surface of the platen 50e using the original-reading device 52, scans the entire surface of the original using the first mirror unit 50a moving from the left to the right, and discharges the original onto the discharged-sheet tray 52b. 
The image scanned by the first mirror unit 50a is guided to the CCD 51 via the second mirror unit 50b and the lens 50c, is converted into electronic data by the CCD 51c, and the electronic data is transmitted to the printer unit 20.
The printer unit 20 transfers images made of necessary ones of a magenta toner, a yellow toner, a cyan toner and a black toner onto a sheet fed from the sheet feeding unit 40 in a superposed state, to provide a color image. The details of a transfer process in a case of using all of four colors will now be described.
The printer unit 20 first causes the magenta developing unit 34a to face the photosensitive drum 30 by rotating the rotating developing member 34.
Then, the photosensitive drum 30 and the intermediate transfer belt 35 are rotatably driven at the same constant circumferential speed by a driving source.
After uniformly charging the surface of the photosensitive drum 30 by the charging means 32, a laser beam 33f from the optical scanning device 33 is projected onto the surface of the photosensitive drum 30 to form an electrostatic latent image for a magenta color. This electrostatic latent image is developed as a magenta toner image by receiving a magenta toner from the magenta developing unit 34d, and is transferred onto the intermediate transfer bent 35.
Magenta toner particles remaining on the photosensitive drum 30 without being transferred onto the intermediate transfer belt 35 are cleaned by the cleaner 31.
After thus completing development of a magenta image, the rotating developing member 34 rotates to dispose the cyan developing unit 34a at a position facing the photosensitive drum 30. A cyan toner image is transferred onto the intermediate transfer belt 35 so as to be superposed on the magenta toner image in the same procedure as for the magenta toner image. Then, the yellow developing unit 34c and the black developing unit 34d are caused to sequentially face the photosensitive drum 30, and a yellow toner image and a black toner image are formed on the intermediate transfer belt 35 so as to be superposed on the previously transferred toner images.
After transferring the four color, i.e., magenta, cyan, yellow and black, toner images transferred on the intermediate transfer belt 35 in the above-described manner onto the sheet fed from the sheet feeding unit 40 as a recording material, toner particles remaining on the intermediate transfer belt 35 are scraped by causing the belt cleaner 35a to contact the intermediate transfer belt 35.
After transferring the color toner image onto the sheet, the printer unit 20 fixes the color toner image on the sheet using the heating fixing unit 36, and then discharges the sheet onto the discharged-sheet tray 45 to terminate the operation.
Next, the conventional fixing unit 36 will be described in detail. As shown in FIG. 4, the fixing unit 36 is disposed near the exhaust fan 55.
As shown in FIG. 5, the fixing unit 36 includes a pair of a fixing roller 61 and a pressing roller 62, each provided by forming a rubber layer on the surface of a core roller. By pressing the pressing roller 62 against the fixing roller 61, a nip portion 63 is formed. The fixing unit 36 is detachably mountable in the main body of the image forming apparatus.
A heater 64 is disposed inside the fixing roller 61, and the fixing roller 61 is maintained at a set temperature by a control device.
When the fixing unit 36 is operating, the sheet conveyed from below is conveyed by being grasped at the nip portion 63 between the fixing roller 61 and the pressing roller 62, and the toner image is fixed by being heated by the fixing roller 61.
Case members 65 and 66 are disposed along the surfaces of the fixing roller 61 and the pressing roller 62, respectively, with an interval of 2–3 mm from the surfaces of these rollers. Intervals between the fixing roller 61 and the pressing roller 62, and the case members 65 and 66 are termed interval portions 67 and 68, respectively.
The case members 65 and 66 are made of a resin material having a high heat insulating property. By thus covering the fixing rollers 61 and the pressing roller 62 with the case members 65 and 66, respectively, with a small interval, it is possible to efficiently perform heat insulation for the fixing roller 61 and the pressing roller 62. As proposed in Japanese Patent Application Laid-Open No. 2000-187408, heat insulation for the fixing roller 61 and the pressing roller 62 can be improved by using the case members 65 and 66, respectively.
However, when the fixing roller 61 and the pressing roller 62 are covered with the case members 65 and 66, respectively, with a small interval, air is also sucked from the fixing unit 36 by the exhaust fan 55 provided near the fixing unit 36. Since a space for an entrance to the fixing nip portion for the recording material, a sheet discharge outlet, and the like is necessary in the fixing unit 36, it is difficult to completely close the fixing unit 36. Accordingly, for example, air enters from the entrance, and flows to the outlet through an interval between the fixing roller 61 and the heat insulating material.
As described above, since air present near the surfaces of the rollers 61 and 62 directly moves at interval portions 67 and 68, temperature tends to change at the surfaces of the rollers 61 and 62, resulting in cooling of the fixing roller 61 and the pressing roller 62.
By providing another ventilation channel near the heat insulating material, movement of air at the interval between the fixing roller 61 and the heat insulating material can be reduced by increasing movement of air at the ventilation channel.
However, in order to provide such an another ventilation channel in the main body of the image forming apparatus when mounting the fixing unit, it is necessary to newly mount a plurality of components in the image forming apparatus, resulting in an increase in the number of components in the main body of the image forming apparatus, and a complicated configuration.